The Robert H. Lurie Comprehensive Cancer Center Mouse Phenotyping Core Facility provides comprehensive gross and histopathologic assessment of murine phenotypes, histological evaluation of tissue derived from animal tumor models, and training sessions for investigators to learn to detect gross anomalies in mice, harvest tissue from various organ systems, and to perform immunohistochemical or special stains on tissue sections. This Core Facility was developed to fulfill a need by the investigative community to accurately analyze their new murine models and to enhance the ability to extract meaningful phenotypic information to guide future investigations. Murine tissue has histological characteristics distinct from human tissue;therefore, assessment under the microscope requires pathologists with experience in murine histology. Moreover, tissue harvesting, processing, and sectioning demands precision, especially in the case of embryonic lethal phenotypes. The Core Facility is directed by a perinatal pathologist with more than fifteen years of experience in animal tissue assessment, immunohistochemical analyses, and pathology. Her extensive background in developmental anomalies allows for accurate assessment of embryonic lethal phenotypes and for establishing the proximate cause of the demise. Other team members of this facility include a second perinatal pathologist, a molecular biologist, and a certified histotechnologist. Together, this team provides invaluable expertise to investigators by offering insight into phenotypes of newly developed murine models, by providing training sessions in tissue harvesting and analysis, and by assisting with performance and grading of immunohistochemical studies. This comprehensive approach to phenotyping viable and embryonic lethal models has the potential to discover new target organ systems and signaling partners for molecules and to identify innovative model systems to study cancer biology for the scientific community.